ABSTRACT
These lines correspond to the recombination of an electron-hole pair in the
ground state in the presence of other charges in the QD and the shift in
energy of the new transition lines is due to Coulomb interactions, including
exchange and correlation, between the carriers. At 59nW the spectrum
becomes more complex and both the X and BX lines reach a maximum
and saturate. Increasing the excitation power by about 2 orders of magni-
tude no significant change is observed in the spectra except for an increase
in the background emission. In Fig. 16.7b are plotted the integrated PL
intensities of the X and BX lines as a function of the excitation intensity.
At low excitation power, the PL intensity dependence on the laser power
P can be fitted by the relation IX,BX ∝ Pn, with n = 0.70 ± 0.05 and 1.35±0.05 for the X and BX lines, respectively. (The uncertainty on the fit
having been estimated from fitting the data from several QDs.) The fact
that the ratio of the exponents is equal to 2 (nBX/nX = 2) suggests that
the BX line corresponds to the biexciton emission: the recombination of an
exciton in the ground state of the QD in the presence of another electron-
hole pair in the ground state. The PL intensity ratio BX/X greater than 1
could be explained by the dark X state acting as a channel for depleting the
exciton state, either by non-radiative decay or by allowing additional time
for charging of the dot (following BX emission) and subsequent emission at
a different energy.
